In recent years, mobile devices such as mobile phones have come into widespread use, and higher speed and higher frequency semiconductor elements have been used as main components of personal computers. To support the high frequencies in a small package, there has been a growing demand for smaller multilayer ceramic capacitors having higher capacitances for use in these electronic devices. These multilayer ceramic capacitors are therefore required to include a larger number of dielectric layers having a smaller thickness.
Dielectric ceramics for dielectric layers of multilayer ceramic capacitors may include barium titanate as a main component. Crystal grains having a core-shell structure have recently been developed for dielectric ceramics and are used in practical multilayer ceramic capacitors.
The term “core-shell structure” of a crystal grain, as used herein, refers to a structure in which an inner core and an outer shell of the crystal grain form physically and chemically different phases. In a crystal grain comprising barium titanate as a main component, the inner core has a tetragonal crystal structure and the outer shell has a cubic crystal structure.
A dielectric ceramics comprising crystal grains having such a core-shell structure can have a high relative dielectric constant with a stable temperature dependence. However, as a direct-current voltage applied to the dielectric ceramics increases, an insulation resistance of the dielectric ceramics decreases (hereinafter referred to as the voltage dependence of insulation resistance).
In a multilayer ceramic capacitor that includes ceramic dielectric layers formed of such core-shell crystal grains, such an increased voltage dependence of the insulation resistance of the dielectric ceramics makes it difficult to improve the life of the multilayer ceramic capacitor in a high-temperature durability test.
Accordingly, there is a need for a dielectric ceramics that has a high dielectric constant, a relative dielectric constant with a stable temperature dependence, and an insulation resistance with a reduced voltage dependence. Furthermore, there is a need for a multilayer ceramic capacitor that has high-temperature durability and a long operating life.